Answer:
[tex]\boxed{\text{A. 110 jazz, 440 rock}}[/tex]
Step-by-step explanation:
Let r = the number of rock CDs
and j = the number of jazz CDs
You have a system of two equations:
[tex]\begin{cases}(1) & r = 4j\\(2) & r + j = 550\end{cases}\\\begin{array}{lrcll} & 4j + j & = & 550 &\text{Substitited (1) into (2)}\\& 5j & = & 550 &\text{Combined like terms}\\(3) & j & = & 110 &\text{Divided each side by 5}\\& r & = & 4 \times 110 &\text{Substituted (3) into (1)} \\& r& = & 440 & \text{Substituted (5) into (1)}\\\end{array}\\\text{There are $\boxed{\textbf{\110 jazz, 440 rock}}$ CDs}\\[/tex]
Check:
[tex]\begin{array}{rclcrcl}440 & = & 4 \times 110 & \qquad &440 + 110&=&550\\440 & = & 440 & \qquad &550& = &550\\\end{array}[/tex]
OK.
